The present invention relates to a constant level slurry tank system in particular for use in coating wax patterns for use in investment casting with a ceramic shell by repeated dipping in the slurry.
The manufacture of high-precision engineering components, such as turbine blades and vanes, by investment casting generally includes the step of forming a ceramic shell on a wax pattern of the final component. This is usually achieved by dipping the pattern into a slurry of the ceramic material, drying the coating and then repeating this procedure until the desired thickness of ceramic coating has been built up. Increasingly, this step is being carried out by automatic machinery or robots.
In many instances, it is important that the patterns are dipped to the correct depth. This is particularly so in the case of turbine blades, the wax patterns of which are generally first joined together into a "tree" to enable a number of blades to be formed in one casting operation. If the tree is not dipped in far enough, the upper parts of the patterns will receive no coating. If the tree is dipped in too far, the conical "pour hole" for the molten alloy will loose its clean complete upper edge.
When the dipping is carried out under manual control, the correct dipping extent can be achieved by the operative, regardless of the depth of the slurry, by experience. However, when the dipping is carried out by robots it is necessary either to vary the dipping distance to take into account changes in slurry level or to operate a constant dipping distance and maintain a constant slurry level. Clearly, it is preferable to avoid using a varying dipping distance due to the complicated control system that this would entail. At the same time however, it would be preferable to avoid the use of a complicated slurry level sensing arrangement and associated slurry feed system. This could of course be achieved by means of a float-operated level control but would suffer the disadvantage that its operation would be intermittent. As a result, the level would constantly fluctuate and there would be a risk of the delivery device blocking up when not in operation.